Gasification is a process that converts carbonaceous materials, such as coal, petroleum, or biomass to predominantly carbon monoxide and hydrogen (syngas) by reacting the carbonaceous material at high temperatures under gasification conditions. Syngas may be burned directly in internal combustion engines, used to produce methanol, dimethyl ether, or hydrogen, or converted via the Fischer-Tropsch process into synthetic fuels. Syngas can also be used to produce other products.
Gasification of fossil fuels is currently widely used to generate electricity. A wide variety of carbon-containing material can be used as the feedstock for gasification, including biomass and plastic waste. Thus, gasification has the potential to be an important technology for renewable energy, and is generally carbon neutral.
Since gasification is an endothermic reaction, heat must be supplied to the carbonaceous material either indirectly through exchange with a hot heat transfer surface or directly through the simultaneous heat release associated with partial oxidation due the introduction of oxygen-containing gas into the reactor. Most carbonaceous material derived from biomass contain significant amounts of inorganic material (i.e. silica, potassium and other elements) that do not undergo gasification and can agglomerate and fuse into a phase commonly referred to as slag when exposed to elevated temperatures (typically >1800 F.). Gasifiers that are designed to minimize slag formation and use partial oxidation to generate the required thermal energy for gasification (directly heated gasifiers) must control the addition of oxygen in order to avoid excessive temperatures within the partial oxidation zone.
The direct injection of oxygen or air into a gasifier chamber typically leads to high temperatures within the gas jet region associated with the nozzle or injection device used to introduce the oxygen. Examples of conventional gasifier designs, or systems, are described in U.S. Pat. Nos. 6,613,111 and 6,680,137 which utilize two fluid bed reactors, each containing a bed of fluidized inert (circulating) and carbonaceous solids. Gasification occurs within one bed (first fluidization stage) and the solids stream comprised of an inert fluidization solids fraction and a carbonaceous (carbon-rich) fraction are collected and routed to a second fluid bed (typically a combustion stage) where they undergo oxidation to raise the temperature of the fluidization solids. The heated inert solids are then sent back to the gasifier section (first stage) supplying heat to the gasification stage. In this type of design, the amount of heat generated in the combustion stage is critical since it must be sufficient to maintain the desired gasification temperature. If the rate of oxidation is excessive (too much carbonaceous material with air), the solids may undergo an unacceptably high temperature rise, resulting in either slag formation which can lead to loss of fluidization or the volatilization and redeposition of undesired inorganic material in the colder sections of the process unit, such as in gas conduits. Conversely, operating the combustion stage at lower than desired temperatures can lead to the accumulation of carbonaceous material in both the gasification and combustion stages of the gasifier system.
Either full or partial oxidation of the carbonaceous material within the second stage can occur. The most important objectives in the combustion stage include the removal of carbonaceous material to prevent accumulation and to generate an effective amount of thermal energy to drive the gasification reactions.
Gasifier designs based on indirect heating by use of hot heat transfer surfaces are best represented by U.S. Pat No. 5,059,404, U.S. Pat. No. 5,306,481 and related patents. In such gasifier systems, the heat required for driving the gasification reactions occurs by use of hot heat transfer tubes located within the fluidized bed.
While there is much activity in the field of gasification, especially for converting biomass to fuel products, there is still a need in the art for improved and more efficient processes and equipment.